Title:X-Ray, UV and Optical Observations of Classical Cepheids: New Insights into Cepheid Evolution, and the Heating and Dynamics of Their Atmospheres

Abstract: To broaden the understanding of classical Cepheid structure, evolution and
atmospheres, we have extended our continuing secret lives of Cepheids program
by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) /
cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids
Polaris, δ Cep and β Dor. Previous studies made with the
international ultraviolet explorer (IUE) showed a limited number of UV emission
lines in Cepheids. The well-known problem presented by scattered light
contamination in IUE spectra for bright stars, along with the excellent
sensitivity & resolution combination offered by HST/COS, motivated this study,
and the spectra obtained were much more rich and complex than we had ever
anticipated. Numerous emission lines, indicating 10^4 K up to ~3 x 10^5 K
plasmas, have been observed, showing Cepheids to have complex, dynamic outer
atmospheres that also vary with the photospheric pulsation period. The FUV line
emissions peak in the phase range φ ~ 0.8-1.0 and vary by factors as large
as 10x. A more complete picture of Cepheid outer atmospheres is accomplished
when the HST/COS results are combined with X-ray observations that we have
obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to
date have X-ray luminosities of log Lx ~ 28.5-29.1 ergs/sec, and plasma
temperatures in the 2-8 x 10^6 K range. Given the phase-timing of the enhanced
emissions, the most plausible explanation is the formation of a
pulsation-induced shocks that excite (and heat) the atmospheric plasmas
surrounding the photosphere. A pulsation-driven α^2 equivalent dynamo
mechanism is also a viable and interesting alternative. However, the tight
phase-space of enhanced emission (peaking near 0.8-1.0 φ) favor the shock
heating mechanism hypothesis.